1. Field of the Invention
The present invention relates to a high-polymer optical low-pass filter, a method for producing a high-polymer optical low-pass filter, and a digital camera having the high-polymer optical low-pass filter.
2. Description of the Related Art
In an electronic image-capturing apparatus such as a digital still camera or a video camera, a subject image is captured by utilizing a solid image-capturing element (or sensor) that may be a CCD element or a MOS element. In such an image-capturing element, light-receiving pixels are arrayed in a regular pattern, and thus, the light-receiving pixel array pattern and the pattern of the subject image tend to cause a moire fringe, color artifacts or the like. Normally, these undesirable effects are eliminated by providing an optical low-pass filter in front of the image-capturing element.
The optical low-pass filter utilized in this application may be manufactured by, for instance, cutting out quartz or lithium niobate (LiNbO3) at a constant thickness along a specific direction. Such an optical low-pass filter takes advantage of the index ellipsoid structure of the crystal and effectively uses the birefringence achieved by tilting the crystallographic axis by a predetermined degree relative to the optical axis. When quartz is used to manufacture the optical low-pass filter, the quartz is cut diagonally at approximately 45 degrees relative to the optical axis. In this case, double images of the subject caused by the birefringence are projected onto the image-capturing element and the spatial frequency component corresponding to the separation width of the two images is cut off. The occurrence of a moire fringe can be prevented by adjusting the cut-off frequency to the light-receiving element array frequency.
However, the inorganic crystal such as quartz or lithium niobate that is used to constitute the optical low-pass filter manifests a piezoelectric effect as well as the optical anisotropy described above. For this reason, even a slight stress applied to the optical low-pass filter causes an electrical charge in the optical low-pass filter generating static electricity, which attracts dust suspended in the air to the filter surface. When the optical low-pass filter is disposed immediately in front of the image-capturing element, which is often the case, the image-capturing element captures an image of dust adhering to the optical low-pass filter as well as the subject image. Since the size of each pixel at the image-capturing element is smaller than 10 μm on each side, dust adhering to the optical low-pass filter which exceeds several tens of μm in size is bound to affect the image.
In particular, in a single-lens reflex type digital camera adopting a structure having a quick return mirror and a shutter provided immediately in front of the optical low-pass filter, the sliding gears and shutter blade generate particles from the coated surfaces, the metals and the like, and these particles become attracted to the surface of the optical low-pass filter having become charged with static electricity. In addition, if the surface of the optical filter is rubbed with a cloth or the like in order to remove the dust, a further electrical charge is induced and, as a result, the optical filter surface cannot be left in a dust free state.
A high polymer material may be used to constitute the optical low-pass filter as a way of preventing such an electrostatic charge. For instance, there is a birefringence type high-polymer optical low-pass filter in the known art achieved by using a diagonally oriented liquid crystal, as disclosed in Japanese Laid-Open Patent Publication No. 2001-75054.
However, when forming an optical low-pass filter by diagonally orienting a liquid crystal, the liquid crystal is oriented through rubbing or the like and thus, it is difficult to accurately orient the liquid crystal along a desired direction. In addition, there is a problem in that it is relatively difficult to achieve a uniform orientation of a liquid crystal having a sufficient thickness.